In view of excellent physical properties such as high heat resistance, moisture proof, dimensional stability, etc., epoxy resin compositions each containing an epoxy resin and a curing agent therefor as essential components are widely used for electronic components such as a semiconductor encapsulate, a printed circuit board, a build-up substrate, and resist ink, a conductive adhesive such as a conductive paste and other adhesives, a liquid sealing material such as an underfill, a liquid crystal sealing material, a cover lay for a flexible substrate, an adhesive film for build up, a coating material, a photoresist material, a color developing material, a fiber-reinforced composite material, and the like.
Among these, particularly, fiber-reinforced resin moldings produced by impregnating reinforcing fibers with an epoxy resin and a curing agent as matrix components and then curing the resin are highly required in general industrial fields such as automobile industry and aerospace industry from the viewpoint of various excellent performances such as high heat resistance, strength, low curing contraction percentage, chemical resistance, high elastic modulus, etc. in addition to properties such as light weight and high strength.
However, epoxy resins are generally high-viscosity fluids or solids at normal temperature, and thus in a step of impregnating fiber reinforcements with the resins, it is necessary to heat resin components in order to secure a practical level of fluidity of the epoxy resins, thereby causing the problem of accelerating curing of the epoxy resins by heating and rather bringing about higher viscosity and impregnation failure. In particular, in a molding technique by a resin transfer molding (RTM) method which has recently been being popularized in the field of carbon fiber-reinforced thermosetting plastics (CFRP) because of the overwhelming cycle time and low equipment cost, low viscosity and high fluidity are important problems for thermosetting resin materials from the viewpoint of higher-cycle molding.
As means for improving fluidity of an epoxy resin material for a CFRP matrix, there has been known a technique of preparing a liquid composition by mixing an aliphatic epoxy compound such as 3,4-epoxycyclohexymethyl-3,4-epoxycyclohexane carboxylate or polyglycidylamine such as N,N,N′,N′-tetraglycidyldiaminodiphenylmethane with acrylic acid, styrene, and a radical polymerization initiator, impregnating a carbon fiber substrate with the liquid composition, and then effecting reaction between epoxy groups and acrylic acid and radical polymerization by heating, producing a molded product (refer to PTL 1 below).
However, when the aliphatic epoxy compound is used for the liquid composition described in PTL 1, a cured product become brittle and thus does not exhibit satisfactory strength, while when the polyglycidylamine is used, heat resistance is not satisfactorily exhibited. In addition, the epoxy resin is a special epoxy resin having excellent curability with acrylic acid and is difficult to produce on an industrial scale and is lack of practicability.
On the other hand, there has been known a technique for improving CFRP productivity by the RTM method, in which as an epoxy resin material suitable for the RTM method for CFRP application, a bisphenol F epoxy resin having an epoxy equivalent of, for example, 200 g/eq. or less, is used as a base resin, and aromatic polyamine, which is liquid at room temperature, and a complex of a Lewis acid and a base are used as curing agent components, thereby improving fluidity of a thermosetting resin component and further improving low-temperature curability (refer to PTL 2).
However, in the thermosetting resin material containing the bisphenol F epoxy resin having an epoxy equivalent of 200 g/eq. or less, the aromatic polyamine which is liquid at room temperature, and the complex of a Lewis acid and a base, the viscosity of the epoxy resin is decreased, but the viscosity of the whole composition is still high, thereby necessitating heating at about 100° C. for resin injection in, for example, RTM molding. Therefore, the possibility of thickening by curing reaction remains, the running cost is increased in terms of energy, and the molding cycle time cannot be sufficiently shortened. In addition, a cured product has unsatisfactory heat resistance and has difficulty in applying to the automobile industry and the aerospace industry.